No field or variable is needed to show the row number.
Enter layout mode
Select the text tool and click the approximate location where you want the row number
Then pull down the Insert menu and select the option for the record number symbol
Now adjust the final position of this text to place it where you want it.
In later versions of FileMaker the text produced by this may have to overlap other fields in the portal row. This will not matter in browse mode. Just make sure that the Portal "owns" this added layout text by moving the portal a few pixels and confirming that this layout text moves with it.
With these later versions, you can resize the text inside the brackets to be a very small font size and it won't affect the size shown when you enter Browse mode so this can be a way to reduce the text inserted to a smaller and easier to manage size.